The invention relates to a method and system for controlling the moments of firing of marine seismic sources which are towed as a seismic array behind a towing vessel.
The art of marine surveying by the use of seismic waves has achieved considerable importance and various methods and systems for conducting marine seismic surveying operations are well known.
It has now become common seismic practice to tow a marine seismic array comprising a plurality of seismic sources behind a towing vessel. The seismic array ray also consist of at least two subarrays, each subarray carrying seismic sources.
In operating marine seismic sources, the seismic sources are fired by a controlling means and after a certain time delay seismic pulses are created. The seismic signals thus obtained are detected and processed further in any way suitable for the purpose.
When operating a plurality of marine seismic sources which are towed as a seismic array behind a towing vessel, the seismic sources can be fired in such a way as to increase the energy, improve directional properties and/or the quality of the seismic pulse needed in a seismic operation.
Another possibility of operating a plurality of seismic sources in a seismic array when applying wave generating techniques is the time-delayed firing of one or more seismic sources with respect to the firing of the other sources.
Usually this time-delay is in the order of 1-100 milliseconds.
In order to obtain an optimal and stable acoustic output of the array of sources it is desirable that the emitted seismic array signal which can be considered as a composition of the acoustic output of the respective individual seismic sources, has an optimum amplitude.
However, according to a generally known theoretical formula the dominant period of a seismic source signal in general is dependent on the source depth and in carrying out marine seismic operations with seismic sources of the explosive and implosive source type, for example air guns, water guns etc. it has appeared that the emitted seismic array signal is influenced by the depths at which the respective individual seismic sources of a seismic array are towed.
Due to several known reasons, for example wave action, speed variations etc. these source depths are not always the same in seismic operations. Thus, it appears that firing a plurality of seismic sources when towed at different depths and summing the obtained individual seismic signals will not give an optimum result as time shifts and differences in individual dominant periods are present.
Thus, it is desirable to synchronize the seismic sources in order to arrive at a certain wave front action by adjusting the delay between the moment of firing, or firing time, of the respective seismic source and the occurrence of the first peak of the seismic pulse, taking into account the seismic sources, instantaneous depths in determining the (electrical) firing time.
Seismic source signal synchronization as applied so far uses means for detecting the time at which an acoustic signal occurs, measured close to the seismic source. The timing history of previous shots is used to control the firing moment which is preformed by a controlling means for the seismic sources. This method, however, only compensates for average timing errors. These and other limitations and disadvantages of prior art are overcome and improved methods and apparatus for controlling the firing times of marine seismic sources are provided.